The invention relates to electronic systems for informing the operator of a motor vehicle of the current direction of travel.
Any such system must deal in some way with errors caused by the magnetic field of the vehicle chassis itself as well as with errors occurring due to naturally-occurring deviation between magnetic north and geographic north in different parts of the world.
Early mechanical compass devices in which the magnetic sensor consisted of a free-floating compass needle used small permanent magnets near the magnetic sensor whose direction or proximity could be manually adjusted in an attempt to cancel the permanent static magnetic field originating in the chassis near the sensor. These early devices which were intended for installation by the user were limited to mounting locations which are relatively free of chassis interference, such as the front windshield or the instrument panel. These mechanical compass instruments have served as the backbone of the automotive "aftermarket" (user installed compass devices) for several decades and remain the most widely used today.
During this time, the aircraft industry began using a radically different form of magnetic sensor that is now generally called a "Flux-Gate Sensor". It consisted of a toroidal-shaped magnetic core with a single primary winding which was driven with an alternating current. Two secondary windings were formed in linear fashion over the core subassembly and in quadrature with each other such that, in the absence of an external fixed magnetic field, the magnetic coupling from primary to secondaries would be balanced and the output voltage of both secondaries would be zero. Any external magnetic flux parallel to the plane of the toroid would tend to reinforce the alternating flux due to the primary current on one half of the toroid and counteract the alternating flux on the other half. The resulting magnetic unbalance produces an output voltage in either or both secondary windings. Output is a maximum when the external field direction is exactly perpendicular to a given secondary winding and becomes zero when it is exactly parallel. If the sensor is rotated by 180 degrees, both secondaries will present an equal output but of opposite polarity relative to the primary drive signal. If the sensor is aligned in the vehicle such that one secondary winding is parallel to the direction of travel, then that winding is designated as the east/west output and the other winding at 90 degrees is the north/south output. The two outputs can be interpreted as representing the E/W and N/S components of the earth's magnetic field and the actual heading angle computed using simple trigonometry.
The development of the Flux-Gate Sensor provided two major improvements over the floating, permanent magnet type:
1. The sensor could now be installed at a location different from the remainder of the system and connected by cable to the location of the other system components.
2. The two secondary windings could serve as electromagnets, magnets, effectively replacing the adjustable fixed magnets to cancel fixed chassis components. Small direct currents of proper polarity could be passed through both windings independently to oppose the chassis components of the vehicle.
At this point in time, the calibration process was still done manually much the same as in the case of the mechanical compasses except that user adjustment involved the setting of variable resistors instead of mechanical screws.
The first automotive aftermarket compass devices with a Flux-Gate Sensor which appeared about four years ago did not take advantage of its remote-mounting capability. This device consisted of a display/control unit mounted to the instrument panel just as before and a sensor unit on a short cable. The sensor was mounted on a horizontal surface of the instrument panel close to the windshield. The main unit included two adjusters to set secondary direct current. Direction of travel was displayed by a rotating disc controlled by a servo motor.
At this time microprocessor technology became widely used for automotive accessories such as the electronically-tuned AM/FM radio with digital display of station frequency and with favorite-station preset data stored in the non-volatile memory area of the microprocessor, such technology effectively replacing mechanical AM/FM tuners having station preset buttons. Following these lines, electronic compass systems also made their appearance in which in the calibration digital displays were used and in which the calibration procedure was automated by a microprocessor, with storage of the calibration result in a non-volatile memory. One such electronic compass system has been disclosed, for example, in U.S. Pat. No. 4,546,551, issued Oct. 15, 1985.
Compass devices using this technology appeared several years ago as factory-installed options in some new American cars. Their application has been strictly limited to one or two models whose configuration lends itself to a custom installation of a single package including sensor in a favorable location such as in the headliner just behind the windshield or as an attachment to the rear-view mirror. Typically, the compass option is only offered in vans or mini-vans.
All known digital compass devices currently available suffer from certain weaknesses that for all practical purposes prevent application to the aftermarket in which the end-user would purchase the device for his currently-owned vehicle and either install it himself or hire a professional installer whose expertise is likely limited to car stereo, burglar alarms and radar detectors. Among these weaknesses are:
1. The calibration procedure requires that the vehicle be prealigned to magnetic north rather than true geographic north. As a factory-installed option, the new car dealer might easily have alignment markers painted on his premises and perform the calibration before delivery. The aftermarket customer has no knowledge of magnetic direction in his area and is faced with the prospect of buying a conventional compass and then trying to align his sixteen foot vehicle to a one inch needle.
2. Workable sensor locations are limited to the windshield or roof area where chassis components are known to be low-typically less than the magnitude of the earth's magnetic field. In the context of a factory-installed option available in only a few limited models, such locations are satisfactory because the hardware can be integrated into the car interior for a pleasing cosmetic effect and the location is also convenient to the operator and passengers. Such desirable locations are generally not available to a viable aftermarket compass for a variety of reasons--both cosmetic and functional. The sensor must be rigidly fixed in a horizontal plane with the secondary winding axes properly aligned to the vehicle. Any movement after calibration destroys its validity. This eliminates the possibility of clipping the sensor to the sunvisor as commonly done with radar detectors. The sensor could be fixed to the top surface of the instrument panel as done previously but this location is subject to interference from other on-board electrical systems or cables and is generally unsightly.